As engines age, carbon buildup inside them can cause engine compression ratios to increase. As a result, spontaneous combustion (such as pre-ignition) and/or knock can occur more frequently. Engines may be configured with a knock sensor coupled to the engine block for detecting engine knock. The knock sensor may be a piezoelectric device that outputs a voltage (e.g., in the range of 5-18 khz) upon excitation. The sensor uses engine vibrations for excitation and does not need external power supply. In response to knock detection by the sensor (such as when the sensor output is higher than a knock threshold), an engine controller may adjust (e.g., retard) spark timing to reduce the likelihood of further knock. To ensure timely knock detection and mitigation, knock sensors may be periodically diagnosed.
Various approaches have been developed to diagnose knock sensors. One example approach shown by Bizub in EP 3054134 A1 relies on comparison of knock sensor to crankshaft speed data. In particular, one or more engine knock events are derived from the knock sensor data, and then it is determined whether the one or more knock events actually took place at a known time or a known crankshaft position. Another approach shown by Homer et al. in U.S. Pat. No. 7,562,558. Therein, a signal generator generates and applies a test signal with a predetermined frequency to a knock sensor. Based on an amplitude of the test signal, as detected by a circuit associated with the knock sensor, a controller determines if the knock sensor's circuit is open or short.
However, the inventors herein have identified potential issues with such approaches. The approach of Bizub relies on the inherent occurrence of engine knock for knock sensor diagnosis. However, since engine knocking occurs more often with an increased resulting compression ratio, or in the presence of elevated temperatures, the knock sensor diagnosing opportunities may be limited. For example, knock may not present itself until the engine is being operated at high load conditions such as during highway driving or towing applications. The knock sensor diagnosing opportunities may be even more limited in city drive cycles and engines with start/stop applications. As a result, there may be situations where the knock sensor needs to be diagnosed (such as to meet OBD requirements), but cannot be diagnosed at that time due to lack of knock generating engine conditions.
In the approach of Horner, even though the knock output is induced via the test signal, the test signal can only be applied during conditions when the knock output induced via the test signal does not interfere with knock output due to actual engine vibration. Further, in both approaches, the knock sensor may output a flat zero voltage if there is no natural vibration from engine combustion. It may not be clear if the zero output is due to a degraded sensor or due to lack of physical knock. If the knock sensor is not timely diagnosed, knock may go undetected and/or the engine controller may adjust the spark timing in a less than optimum manner, causing engine performance issues, fuel economy loss, and elevated tailpipe emissions.
In one example, the issues described above may be addressed by a method comprising: inducing a knock event in a cylinder via operation of a laser ignition device; and diagnosing a knock sensor based on knock sensor output following the induced knock event. In this way, an engine knock sensor may be reliably diagnosed.
As one example, an engine system may be configured with laser ignition and start/stop capabilities. Responsive to idle-stop conditions being met, the engine may be shut-down and spun towards rest. In particular, the engine may be shut-down with a cylinder located closest to the engine's knock sensor parked in a sealed position when the engine comes to rest. The sealed position may include a position where each of the intake valve and the exhaust valve of the cylinder is closed, such as at a top dead center of an exhaust stroke of the cylinder. While the engine rests at that position, a laser igniter may be operated (e.g., for a duration) to generate heat in the cylinder. If the laser is maneuverable, a beam direction and focal point may be adjusted on different regions of the cylinder (e.g., at random or targeted) so as to generate heat throughout the cylinder. Since both the intake and the exhaust valve are closed, the generated heat is trapped in the cylinder. When engine restart conditions met, engine fueling is resumed while spark timing is advanced for the given cylinder. The combination of trapped heat and spark advance induces knock in the given cylinder during the engine restart. The knock sensor output is observed to determine if the sensor responds to the induced knock. If sensor output is not observed, it may be inferred that the sensor is degraded.
In this way, an engine knock sensor may be diagnosed by inducing knock in an engine cylinder during an engine restart. By using a laser ignition of the engine system to generate and trap heat in a cylinder while an engine is shut-down, existing engine components can be leveraged for inducing knock. By using laser-generated heat and spark advance to induce knock in a cylinder, an engine controller does not need to wait for specific conditions where knock inherently occurs. Consequently, a knock sensor may be diagnosed more frequently, including when required. By diagnosing a knock sensor in a timely and more reliably manner, knock mitigating spark retard can be scheduled more optimally, fuel economy losses can be reduced, and tailpipe emissions can be better controlled.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.